The invention relates to collapsible steering column assemblies of the kind which include a steering shaft that passes through and is supported by a shroud that comprises an outer tube and an inner tube, at least one of which is securely fixed to a part of the vehicle body, the two tubes being free to telescopically collapse relative to each other when a crash force is applied to the steering wheel.
Collapsible Steering column assemblies are known which include a clamp mechanism that when locked prevents unwanted relative movement of the inner and outer tubes of the shroud of the steering column assembly. The clamp mechanism can be unlocked by movement of a locking lever or, sometimes, electrically using a motor or perhaps hydraulically or pneumatically.
By releasing the clamp mechanism and telescopically sliding the outer tube of the shroud over the inner tube the length of the shroud can be altered, altering the reach position of an associated steering wheel. By moving both tubes together up and down relative to the clamp mechanism the rake can be adjusted. In some assemblies only one of reach and rake may be adjusted.
The clamp mechanism may be attached to a bracket (known as a Rake Bracket) which is connected to the vehicle through one or more frangible fasteners. In the event of a crash causing a high load to be applied to the wheel the fasteners can sever allowing the bracket and the clamp mechanism to move relative to the vehicle, in turn allowing whichever part of the shroud it is secured to also to move so the wheel can collapse away from the driver.
To control the rate of collapse it is known to provide an energy absorbing device such as one or more straps which engages the vehicle body at one end and the clamp mechanism at the other. As the bracket moves relative to the vehicle it causes the strap to deform beyond its elastic limit and this deformation absorbs some of the energy, controlling the rate of collapse.
In many collapsible columns, a pair of Energy Absorption Straps (EAS's) is anchored to the fixings blocks (the so-called fusible “Capsules”) via which the steering column's bracket is mounted to a part of the vehicle structure (usually the so-called “cross-car beam”) at its two upper mountings. Each strap passes over (or through) a labyrinthine “anvil” which is fixed to a rake bracket secured to the vehicle body. The EAS, which is generally straight, is deformed locally, in order to navigate the anvil.
Typically, each capsule has a vertical hole at its centre through which passes a downward extending mounting stud which is integral with the vehicle's structure. The capsule is secured to the rake bracket by small in-situ moulded plastic pins which break if there is a severe impact by the driver's torso on the steering wheel. In a crash, the anchored end of each strap remains stationary relative to the vehicle structure while the rake bracket, which incorporates the anvils, moves forwards. This causes the anvil to be dragged along the EAS, progressively deforming and then straightening each element of it. The effect is to absorb the kinetic energy of the driver's upper torso in a progressive manner and to thereby avoid excessive peak forces.